Polytene chromosomes are giant chromosomes found in certain cell types of dipteran flies that form through repeated rounds of DNA replication without cell division. They can reach lengths of 200 micrometers and contain many longitudinal strands called chromonemata. The large size is due to endomitosis, which duplicates the DNA without dividing the cell. Dark bands on the chromosomes contain more DNA and RNA than the lightly stained interbands. Specific regions of the chromosomes can puff out during transcription. Polytene chromosomes are found in the salivary glands and other tissues of flies and allow for high levels of gene expression through multiple copies of genes.

1. POLYTENE CHROMOSOME
Guided by-
Dr. Kaushik ku. Panigrahi
Asst. Prof.(PBG)
Presented by-
Jevelin Swain
(26C/15)

2. INTRODUCTION
• Polytene chromosome are giant chromosome
common to many diptera flies.
• First discovered by E.G Balbiani in 1881 , in of
salivary glands , giving them the commonly used
name salivary gland chromosome.
• These are somatically paired.
• The number of giant chromosome in salivery gland
cell always appear to be half that in normal somatic
cell.

3. • They are occur in rectal epithelium and malphigian
tubules.
• They are many larger than the normal
chromosomes reaching a length of 200
micrometre and are visible even under a compound
microscope.

4. • Because of these chromosome actually consist of
many strands , they are called as polytene
chromosome.
• Polytene chromosome are oversized chromosomes
which have developed from standard chromosome .
• Specialised cells undergo repeated rounds of DNA
replication without cell division , to increase cell
volume , forming a polytene chromosome and the
condition is described as Polyteny.

5. MORPHOLOGY
• The larger size of chromosome is due to presence of
many longitudinals strands called chromonemata.
• The many strands of giant chromosome is due to
repeated division of chromosome without
cytoplasmic division (Endomitosis).
• The polytene chromosome contains two type of
bands namely dark band(80% DNA) and interband
(15% DNA).

6. • The dark bands are darkly strained and inter bands
are lightly stained with nuclear stains.
• The dark bands contain more DNA and less RNA.
• The bands of polytene chromosome become
enlarged at certain times to form swelling called
puffs.
• The formation of puffs is called puffing.

8. • In regions of puffs , the chromonemata uncoil and
open out to form many loops.
• The puffs indicate the sites of active genes when m
RNA synthesis takes place.
• Intensely stained chromosomal segments
correspond to high degree of packing and are
generally inactive (Heterochromatin).
• Less tightly packed segmented stain less distinctly
with genetic activity(Euchromatin).

9. • In Drosophila , 5000 bands have been found in the
4 chromosomes of salivary gland cells.
• Chromosomes in bands are at right angles to the
long axis of chromosome.
• Bands have high DNA content and absorb U.V light.
• Contain 5 long arm and 1 short arm radiating from
a central point called chromocentre , formed by the
fusion of of all the 8 chromosome found in the cell.

10. • The short arm radiating from the chromocentre
represents chromosome IV, one of the long
chromosome is due to X chromosome , while the
remaining 4 long arms represent the arms of
chromosome II and III.

12. FUNCTION
• In addition to increasing the volume of the cells
nuclei and causing cell expansion , polytene cells
may also have metabolic advantage as multiple
copies of genes permits a high level of gene
expression.
• In Drosophilla melanogaster, the chromosomes
of the larval salivary glands under many rounds of
endoreduplication, to produce large amount of glue
before pupation.